We measured total serum bilirubin values in paired capillary and venous samples from 79 untreated jaundiced newborn infants (group 1) and in 29 infants who were receiving phototherapy (group 2). While bilirubin values from the two sites correlated significantly for both groups, capillary samples underestimated venous bilirubin values when the latter exceeded 170 μmol/L (10 mg/dL) (mean and 95% confidence limits: group 1, −15.1 μmol/L [−0.9 mg/dL] and −24.7 to −5.5 μmol/L [−1 to −0.3 mg/dL]; group 2, −10.3 μmol/L [−0.6 mg/dL] and −17.1 to −3.4 μmol/L [−1 to −0.2 mg/dL]). Furthermore, capillary samples underestimated venous bilirubin levels by more than 17 μmol/L (1 mg/dL) in eight of 16 group 1 patients and five of 18 group 2 patients when venous bilirubin values exceeded 170 μmol/L (10 mg/dL). Lower capillary values at higher bilirubin levels might be due to the influence of environmental light. As clinical treatment decisions may be made on the basis of differences in serum bilirubin level of about 17 μmol/L (1 mg/dL) and as capillary samples may underestimate venous bilirubin levels by a similar amount, it may be prudent to measure venous rather than capillary bilirubin levels when the total serum bilirubin level exceeds 170 μmol/L (10 mg/dL). © 1987, American Medical Association. All rights reserved.